The present invention relates to the efficient transmission of power in implantable medical devices, and more particularly to an improvement to inductive coupling for Implantable Cochlear Stimulator (ICS) systems. Such implantable cochlear stimulator systems provide the sensation of hearing for the hearing impaired and the profoundly deaf. Inductive coupling serves the important function of transmitting power from an external device to the implantable device of the cochlear stimulator system. The power is transmitted from a primary coil in the external device, to a secondary coil in the implantable device. The power used in known ICS systems is provided by a battery in an external part of the system, and a larger battery is required if power is wasted. Therefore, efficient inductive coupling is essential for miniaturized systems that are under development.
U.S. Pat. No. 4,400,590 issued Aug. 23, 1983 for xe2x80x9cApparatus for Multi-Channel Cochlear Implant Hearing Aid Systemxe2x80x9d describes and illustrates a system for electrically stimulating predetermined locations of the auditory nerve within the cochlea of the ear, which system includes a multi-channel intra-cochlear electrode array. The hearing aid system described in the ""590 patent receives audio signals at a signal processor located outside the body of a hearing impaired patient. The processor converts the audio signals into analog data signals which are transmitted by a cable connection through the patient""s skin to the implantable multi-channel intra-cochlear electrode array. The cable connection through the skin of the patient to the intra-cochlear electrode array is undesired in that it interferes with the freedom of movement of the patient and represents a possible source of infection.
U.S. Pat. No. 4,532,930, issued Aug. 6, 1985 for xe2x80x9cCochlear Implant System For an Auditory Prosthesisxe2x80x9d describes and illustrates a multiple electrode system which does not employ a through the skin connector, and therefore avoids the problems associated with a cable passing through the skin. The ""930 patent describes an inductive link between a primary coil disposed outside the body, and a secondary coil implanted within the body. U.S. Pat. No. 5,891,183 issued Apr. 6, 1999 for xe2x80x9cDevice for Transferring Electromagnetic Energy Between Primary and Secondary Coils,xe2x80x9d describes coils designed to improve the magnetic coupling between the primary and secondary coils. Different coil configurations are described and modeled in the ""183 patent, and the coupling coefficients for differing coil alignments are presented. The ""930 and the ""183 patents are herein incorporated by reference.
U.S. Pat. No. 5,824,022 issued Oct. 20, 1998 for xe2x80x9cCochlear Stimulation System Employing Behind-The-Ear Speech Processor with Remote Control,xe2x80x9d describes a cochlear stimulator system with a Behind-The-Ear (BTE) speech processor. BTE speech processors offer several advantages, but because of their small size, are limited in the size of the battery they may carry (which in turn limits the useful life of the battery.) The small battery size results in a requirement for very low power consumption.
Unfortunately, in known implantable stimulation systems, the efficiency of the inductive power transmission is reduced by capacitive coupling between turns of the secondary coil and by unwanted magnetic coupling of the secondary coil with surrounding material. The capacitive coupling between turns of the secondary coil is a strong function of the surrounding materials, and the presence of a high dielectric material intensifies the electric field across the turns and terminals of the coils, adding shunt capacitance to the coil. Thus, when a coil is directly immersed in a high dielectric material, or in the proximity of a high dielectric material, such as a human body with a relative dielectric constant eighty one times that of air, the capacitive coupling between individual turns results in the self resonance frequency of the coil being substantially reduced, and the overall mutually coupled inductor circuit operation is greatly changed.
Known implantable devices position the secondary coil in close proximity to other electrical components of the implantable device, or place the secondary coil in a case which contains many of other electrical components of the implantable device. Such placement results in magnetic coupling between the secondary coil and the other electrical components, or with the case. The capacitive and magnetic coupling, plus other losses, results in only about 50% efficiency in inductive power transmission, and therefore necessitates a larger battery to meet power requirements. The need to minimize battery size results in a continuing need for greater efficiency in the inductive transfer of power to implantable medical devices.
The present invention addresses the above and other needs by providing an implantable secondary coil assembly with a spiral shield. The secondary coil assembly includes a winding with a small number of substantially round turns. The winding comprises multi-strand wire in a bio-compatible polymer sheath. A spacer, made from a bio-compatible polymer material, resides between adjacent turns of the coil, and between the outermost turn of the coil and the shield. The spiral shield is electrically connected to the outer turn of the winding, and is wound toroidally around the winding on both sides of the connection, with an electrical gap existing in the shield opposite the connection. A magnet that is about one third to one half the diameter of the assembly is provided at the center of the secondary coil to provide a means for holding a head piece containing a primary coil adjacent to the secondary coil. The entire assembly is preferably encapsulated in the same bio-compatible polymer material used for the sheath and the spacer. In one embodiment, a balum transformer is included which connects the secondary coil to the load.
In accordance with one embodiment of the invention, a spiral shield is wound toroidally around the winding of an implantable secondary coil. The implantable secondary coil receives energy inductively transmitted from an external primary coil. Efficient inductive energy transfer requires that the primary and secondary coil be tuned to the same resonant frequency. An unshielded secondary coil, implanted directly in the human body, experiences operational frequency resonance shifts that result from dielectric loading of the implanted secondary coil. The use of a spiral shield confines the electrical fields applied to the winding, and thereby limits capacitive loading between turns. By limiting the capacitive loading, the operational frequency resonance shifts that result from capacitive loading are minimized, and the secondary coil can be accurately tuned to the resonant frequency of the primary coil, regardless of the presence or absence of other dielectric material, thus stabilizing the tuning and improving the efficiency of the inductive power transmission.
It is a feature of the present invention to provide a spacer made from a bio-compatible polymer material, which spacer is laid between adjacent turns of the secondary coil winding. The bio-compatible polymer material selected has a low dielectric constant, thus reducing turn-to-turn and turn-to-shield capacitances, and making the material an ideal coil potting material. Reducing the turn-to-turn and turn-to-shield capacitances prevents excessive reduction of the self resonant frequency of the coil which might otherwise occur.
It is another feature of the invention to use a spirally wrapped foil, solid or multi-strand wire, or wire mesh as a shield. A solid shield would result in a very stiff structure, which structure would distort or collapse when the coil assembly is flexed. The spiral wrapping provides a very flexible member, similar to a coil spring, which can easily be bent without permanent damage or distortion.
It is a further feature of the invention to encapsulate the secondary coil in a bio-compatible polymer insulation. The combination of the bio-compatible polymer encapsulation, multi-strand wire, and spiral shield, results in a thin, flexible, secondary coil assembly. Such a thin and flexible assembly easily conforms to contours of the skull at the site of an implant, may be located in tissue that experiences flexing, and resists damage from impacts. Further, the wire of the present invention is in a bio-compatible polymer sheath. As a result of employing the same bio-compatible polymer material for the encapsulation, the spacer, and the sheath, the secondary coil assembly has a uniform flex and therefore conforms uniformly to the implant location.
It is an additional feature of the encapsulation of the present invention to allow the secondary coil to be located outside a hermetically sealed metal case, or any case containing other electrical components. The presence of other electrical components in the vicinity of the secondary coil results in unwanted magnetic coupling. Such unwanted magnetic coupling reduces the efficiency of the inductive power transmission. In some embodiments, a permanent magnet is located in the center of the secondary coil as part of a headpiece retention system. In the present invention, the magnet is limited to being about one-half the diameter of the secondary coil. Advantageously, the magnetic fields from the magnet are generally confined within a region of one-half the secondary coil diameter, and centered in the secondary coil. Thus, the magnetic field from the magnet does not couple with the magnetic fields of the inductive coupling, thereby reducing losses from magnetically induced eddy currents.
It is also a feature of one embodiment of the present invention to connect a balum transformer between the secondary coil and the load, to achieve balanced operation of the coil. Balanced operation reduces by one half the voltage present between the secondary coil ends, and the case containing the implantable electronics, and provides a DC-coupled method of attaching the shield to the case. Such balanced operation reduces or eliminates problems resulting from electronic fields in the human body. The balanced configuration also connects the winding-to-shield, and winding-to-case capacitances in series, thus reducing the apparent capacitance shunting the winding. This increases the self-resonance frequency of the coil substantially, which can allow operation at higher frequencies, or alternatively, allows an increase in the number of turns in the coil for operation at a single fixed frequency. Increasing the number of turns increases the coupling between coils such that higher bandwidth, and lower losses, can be obtained.
As a result of the several improvements to the efficiency of inductive power transmission from the primary coil to the secondary coil assembly, recited above, the total power required to operate an implanted medical device from an external power source is substantially reduced. Such reduction in power requirements enables the use of a small battery in BTE hearing devices, and other applications, requiring compact implantable device systems.